The overall goal of this work is to detemrine which neurotransmitters are important in regulating nociception at the level of the spinal cord and the mechanism of the regulations. Evidence from the literature shows that norepinephrine, opioids, and gamma aminobutyric acid are involved in regulation of nociception at the level of the dorsal horn of the spinal cord. Although the mechanisms of this antinociception remain unknown, preliminary evidence suggests that norepinephrine and morphine may act in part by inhibiting the release of the putative afferent neurotransmitter substance P. The purpose of the proposed studies is to determine if norepinephrine, opioid agonists, and gamma-aminobutyric acid inhibit the release of the afferent nociceptive neurotransmitters, substance P, and somatostatin. Peptide release will be induced in superfused rat spinal cord slices using high extracellular potassium and veratridine. Substance P and somatostatin in the perfusate will be measured by radioimmunoassay. Dose-response relationships for norepinephrine, opioids, and gamma aminobutyric acid agonists will be determined. Specific receptor antagonists will be used to determine which receptors mediate any observed inhibition of transmitter release. Studies will also be performed in spinal cord slices taken from rats made tolerant to analgesic doses of morphine to determine if tolerance develops to the inhibition of peptide release. Finally, studies will be performed in rats pretreated with the neurotoxin, capsaicin, to determine if the observed effects on peptide release are occurring on primary afferent neurons. Understanding neurotransmitter interactions at the level of the dorsal horn of the spinal cord is critical in determining what mechanisms regulate nociception and ultimately in designing clinical strategies for the management of pain. The proposed experiments will provide new information regarding the neurochemistry of regulation of afferent neurotransmitter release and will complement previous electrophysiological and immunohistochemical studies.